The present invention relates to a print head assemblies and in particular, to such assemblies which are suitable for use in the rapid sequential printing of packaging materials and/or other relatively flexible, flat stock surfaces.
There are increasingly implemented requirements that mass-produced packaged items carry bar codes, price details, packaging dates and/or "sell by" dates. In the case of dates, it is of course generally neither possible nor practical to integrate them into the basic package details that are printed on the stock of package blanks. Consequently, there exists a need for some means by which supplementary details can be printed onto packaging blanks shortly before they are used, and more frequently, at the point of packaging in light of the risk of either package material waste, or inadvertent use of pre-dated material of the wrong date. It is common knowledge that the majority of all items sold by, for example, all supermarkets in the United Kingdom and United States are individually date stamped. This practice is common to the majority of "consumer societies," and consequently, there exists a substantial need, worldwide, for equipment to apply such information.
Because of the tremendous number of items that are required to be printed with such information, it is, of course, important that the equipment be capable of very rapid and very reliable printing. Unfortunately, as with other industrial machinery, reliability and speed of operation generally have to be traded one against the other and, as a result, performance is often compromised. Consequently, there exist the twinned problems of how to improve operating speeds without a decrease in reliability, and how to improve reliability without reducing operating speeds. An optimum solution to these problems would improve both aspects of performance simultaneously.
Furthermore, in order to avoid the introduction of extra handling steps, it is desirable that printing machinery be readily adaptable to the packing machinery. Consequently, it is desirable for the printing machinery to be compact, self-contained and portable. Unfortunately, much of the conventional printing machinery is very bulky and/or has to be operated in a particular orientation, thus restricting the integration of the printing and packaging machines, as well as limiting the usefulness thereof.
Known print head assemblies that are configured to operate with hot ink tend to be considerably more complex than their conventional, non-hot ink counterparts and, as a result, their reliability may be reduced. In particular, hot-foil machines which use ink-carrying foils analogous to a typewriter's ribbon along with brass type, are disadvantageous because they are bulky, expensive and operate at high temperatures, typically about 130.degree. C. Furthermore, such machines require that precise alignment (often to tolerances less than 1 thousandth of an inch) be maintained between the type and the work surface which is to be printed. Although hot-foil machines are widely used for printing goods packaged with wrapping or film packaging machines, the high temperatures and large printing pressures required frequently damage the packaging films.
It is an object of the present invention to provide an improved print head assembly for use with hot ink, and in which the aforesaid problems and disadvantages of known print head assemblies are mitigated or eliminated.
According to a first exemplary embodiment of the present invention, there is provided a print head assembly comprising: a type holder for carrying one or more type elements (hereinafter referred to merely as "type"); means for reciprocating the print head between a rest position and a printing position; ink supply means, including ink heating means, arranged in operation to supply the type with a heat-softened ink; and type heating means for maintaining the type in a heated condition when the type is in the rest position. The arrangement of components is such that the type carried by the type holder engages the ink supply means during passage of the print head and type holder from the rest position to the printing position, but does not engage the ink supply means on the return passage from the printing position to the rest position.
By using a thermoplastic ink and heated type, it is possible to largely overcome the problem of smeared ink (caused by the ink remaining wet after down time or other printing), even at high operating speeds. Moreover, by ensuring that the ink supply means and the type elements do not come into contact during passage of the type holder from the printing position to the rest position, the life of the ink supply is extended and unwanted build-up of ink on the type is minimized, thereby facilitating the maintenance of clear printing.
In an exemplary embodiment, the means for heating the type are mounted remote from the type holder. An advantage of such an arrangement is that the type can be heated from its printing surface inwardly, helping to minimize warm-up times after breaks in production. The means to heat the type remain stationary during normal operation of the assembly, and this helps to keep the reciprocating mass low, reducing wear and enhancing the overall reliability of the assembly.
In this exemplary embodiment, the means to heat the type may comprise an anvil against which the type rests when the type holder is in the rest position. Such an arrangement provides a compact heat source which facilitates the uniform and quick heating of the whole of the printing surface of the type. Preferably, the anvil includes an electrical heating element which can be removed without disassembly of the print head. This arrangement facilitates quick and cost effective repairs in the event the heating element requires replacement. Moreover, with the heating element held stationary, its associated electrical leads are not subject to repeated flexure, and hence are unlikely to fail by fatigue as in prior art arrangements.
Fluid power means are provided to move the ink supply means and the type holder as explained in greater detail below. The fluid power means preferably comprise air cylinders, facilitating the production of a compact and reliable unit, but other suitable drive means may also be employed. Correct synchronization of the movements of the ink supply means and the type holder is also facilitated by operatively connecting the cylinders for sequential or simultaneous movement.
The ink heating means in accordance with the invention comprises an oven which encloses and heats a supply of heat softenable ink. Use of the oven enables the supply of ink to be heated to a substantially constant temperature from the working surface inwards, and facilitates the use of type having lower thermal mass, which in turn, enables shorter warm-up periods. Print quality is also likely to be more consistent.
In an exemplary embodiment, the ink supply means comprises a substantially cylindrical cartridge including an ink impregnated roll mounted on a spindle or axle. In this exemplary embodiment, the oven substantially surrounds the cartridge and mounts the cartridge for relatively free rotational movement. At the same time, the oven is operatively connected to the above-mentioned fluid power means for movement into and out of the path of travel of the print head as will be explained in greater detail below. Means may also be provided for braking the free rotation of the ink supply roll to ensure more uniform distribution of ink on the roll.
The above explained arrangement of type heating means and ink supply heating means, in conjunction with the use of heat softenable ink (available in a range of colors), permits the use of type constructed of rubber or like material. This is advantageous since the flexibility of the rubber allows greater tolerance, i.e., the type surface and surface to be printed need not be perfectly parallel. In addition, rubber type, unlike metal type, will not perforate the typically thin film surface to be printed, thus eliminating exposure of the package contents, such as food, to the print head, type, ink, etc. which would otherwise require disposal of the package. Rubber type is also substantially less expensive than metal type.
In a related aspect of the invention, the printing unit may be adjustably mounted atop a control box which permits adjustment of the type and ink heating means, as well as the printing speed.
It will be appreciated that the above described print head assembly has several advantages over prior art print head devices including fast start up; easily changeable rubber type elements which will not puncture the printed film surface; lubrication free components which avoid film contamination; and rapid printing speeds of 120 to 200 prints per minute.
Additional objects and advantages of the subject invention will become apparent from the detailed description of the accompanying drawings.